The present invention relates generally to an endpoint detection method and apparatus, and more particularly to a method and apparatus that utilize a chelating agent to detect a polishing endpoint of a semiconductor wafer.
Semiconductor integrated circuits are typically fabricated by a layering process in which several layers of material are fabricated (i) on or in a surface of a wafer, or (ii) on a surface of a previous layer. This fabrication process very often requires layers to be fabricated upon a smooth, planar surface of a previous layer. However, the surface topography of layers may be highly uneven due to (i) areas which are higher than the remainder of the surface or (ii) an uneven topography of an underlying layer. As a result, a layer may need to be polished so as to present a smooth planar surface for the next processing step, such as formation of a conductor layer or pattern on the surface of another layer.
In general, a semiconductor wafer may be polished to remove high topography and surface defects such as crystal lattice damage, scratches, roughness, or embedded particles of dirt or dust. The polishing process typically is accomplished with a polishing system that includes top and bottom platens (e.g. a polishing table and a wafer carrier or holder), between which the semiconductor wafer is positioned. The platens are moved relative to each other thereby causing material to be removed from the surface of the wafer. This polishing process is often referred to as mechanical planarization (MP) and is utilized to improve the quality and reliability of semiconductor devices. The polishing process may also involve the introduction of a chemical slurry to facilitate (i) higher removal rates, and (ii) selective removal of materials fabricated upon the semiconductor wafer. This polishing process is often referred to as chemical mechanical planarization or chemical mechanical polishing (CMP).
In these polishing processes, it is often important to determine an endpoint of the polishing process. Overpolishing (removing too much) of a conductive layer results in increased circuit resistance and potential scrapping of the semiconductor wafer. Since many processing steps have occurred prior to the polishing process, scrapping a semiconductor wafer during fabrication may result in a significant financial loss. Underpolishing (removing too little) of a conductive layer on the other hand leads to failure in isolating circuits and results in electrical shorts, which leads to rework (redoing the CMP process) which raises the cost of production. Thus, a precise endpoint detection technique is needed.
A typical method employed for determining endpoint in polishing systems is to measure the amount of time needed to planarize a first wafer, and then to run the remaining wafers for similar times. In practice this method is extremely time consuming, since operators must inspect each wafer after polishing. The wafers must be inspected because the removal rate may vary during the polishing of an individual wafer, thus making it extremely difficult to precisely control the polishing process.
Another method employed for determining the endpoint in polishing systems is to (i) form a polishing endpoint layer in the semiconductor device, and (ii) polish the semiconductor device down to the polishing endpoint layer. To this end, polishing systems detect when the polishing process reaches the polishing endpoint layer and terminate the polishing process in response to reaching the polishing endpoint layer. Various techniques have been used to detect when the polishing process reaches the polishing endpoint layer. For example, U.S. Pat. No. 5,668,063 to Fry et al. polishes a semiconductor device down to a tracer layer of detectable material. The polishing system of Fry determines that the tracer layer has been reached when a chemical element detector detects materials such as boron or phosphorous of the tracer layer have been removed by the polishing process.
In order to base endpoint detection upon directly detecting material of the tracer layer, the chemical element detector needs to accurately detect rather small amounts of the tracer layer material, or the polishing system needs to remove more of the tracer layer material in order to provide the chemical element detector with enough material for accurate detection. The above is also true if the material of the tracer layer is consumed as a reagent of a chemical reaction to be detected by the detector. In this case, the detector would need to be able to detect the effect of a small reaction, or the polishing system would need to remove more of the tracer layer in order to provide enough tracer material for a substantial reaction to occur.
Detectors capable of detecting small amounts of the tracer layer or detecting the effect of a small chemical reaction are more expensive than detectors capable of detecting larger amounts of the tracer layer or detecting the effect of a larger chemical reaction. Furthermore, the additional removal of the tracer layer in order to provide more tracer layer material for direct detection increases the risk of overpolishing especially when the topography of the tracer layer is highly uneven.
Thus, a continuing need exists for a method and an apparatus which accurately and efficiently detects when a polishing system polishes a semiconductor device down to a polishing endpoint layer.